In recent years, an optical system for forming an image of an object in a space by using a reflective imaging element has been proposed (for example, Patent Documents 1 to 6). The optical system includes a reflective imaging element and an object, and an image to be displayed in a space (referred to as an aerial image) is an image of the object, which forms at a position of planar symmetry with respect to the reflective imaging element as a plane of symmetry.
The reflective imaging element disclosed in Patent Document 1 has a plurality of throughholes penetrating through a plate-like substrate along its thickness direction, such that an optical element composed of two orthogonally-disposed specular elements is formed on the inner walls of each hole (see FIG. 4 of Patent Document 1), or has a plurality of transparent chimneys protruding in the thickness direction of the substrate, such that an optical element composed of two orthogonally-disposed specular elements is formed on the inner wall surface of each chimney (see FIG. 7 of Patent Document 1).
In the reflective imaging elements disclosed in Patent Documents 1, 2, and 5, tens to hundreds of thousands of square holes, each of whose sides measures about 50 μm to 1000 μm, are formed in a substrate having a thickness of 50 μm to 1000 μm, the inner surface of each hole being mirror coated by electroforming technique, nanoprinting technique, or sputtering technique.
Moreover, a reflective imaging element which is disclosed in Patent Document 6 is formed by placing two members, having specular surfaces which are disposed mutually parallel along the longitudinal direction, in overlying relationship in such a manner that the specular elements of the respective members orthogonally intersect (see FIG. 4 of Patent Document 6). Another reflective imaging element has two members, which include numerous elongated grooves that are dug in parallel to one another along the longitudinal direction, and is formed by placing the two transparent substrates in overlying relationship so that smooth surfaces which are formed within the grooves of the respective members for causing total reflection of light orthogonally intersect (see FIG. 9 of Patent Document 6).
The aforementioned optical system in which a reflective imaging element is used utilizes specular reflection or total reflection of the reflective imaging element, and, according to its principles, the ratio in size between the image of the object and the image appearing in the space is 1:1.
For reference sake, the entire disclosure of Patent Documents 1 to 6 is incorporated herein by reference.